1. Field of the Invention
The present invention relates to an imaging apparatus such as a video camera or a still camera, and more particularly, to an imaging apparatus having an autofocus function performing autofocus control based on a video signal obtained by imaging a subject.
2. Description of the Background Art
Conventionally, for an imaging apparatus having the autofocus function, such as a video camera or a still camera, a method has been developed in which a video signal itself obtained from the imaging apparatus is used for evaluating a focus control state, in view of the fact that a picture of a subject will have a higher contrast and a high frequency component of the video signal will be increased as the subject is more focused.
Such a method has a number of excellent characteristics such that no parallax essentially exists, and accurate focusing can be attained even when a depth of field is shallow or when a subject is located far from the imaging apparatus. Furthermore, such a method eliminates the need for additional provision of a sensor specific to autofocusing, and also extremely simplifies the mechanism of the apparatus.
An example of autofocus control using such a video signal includes a conventionally known control method referred to as a so-called hill-climbing servo method.
The hill-climbing servo method is disclosed in, for example, Japanese Patent Laying-Open No. 3-68280, which describes that, in summary, a high frequency component of digital data obtained by A/D converting an imaging video signal within a focus area set at the center of a screen is integrated by each field using an integrating circuit. The obtained digital data of one field is then held as a focus evaluation value, and relative positions of a focus lens and an imaging device are moved such that the focus evaluation value constantly assumes a peak value (i.e. the value is an in-focus position) by always comparing the focus evaluating value with a focus evaluating value in the previous field.
The autofocus control according to the conventional hill-climbing servo method was constituted such that the autofocus control was performed based on an imaging video signal within a relatively small focus area set at the center of the screen, and if no peak value, i.e. in-focus position, of the focus evaluation value was found, to expand the focus area and recommence the autofocus operation based on an imaging video signal within the expanded new focus area.
For example, FIG. 12 schematically shows a method of setting such a focus area on an imaging screen. In FIG. 12, a region 00 illustrated by a grid pattern indicates an entire imaging screen.
In the autofocus operation according to the conventional hill-climbing servo method, a relatively small region consisting of e.g. regions 0 and 5 in FIG. 12 was initially set as a focus area, and if no peak value was found in a series of focus evaluation values obtained from the imaging video signal within the area, a relatively large region further including regions 1, 2, 3 and 4 in addition to the regions 0 and 5 was set as a new focus area to recommence the autofocus control in accordance with a series of focus evaluation values based on the imaging video signal obtained from the entire focus area.
However, such above-described conventional autofocus control based on the series of focus evaluation values obtained from the entire focus area may cause a problem in that a subject cannot be accurately focused.
First of all, supposing that subjects are scattered at different distances within the imaging region, it is common, in taking a picture with a camera or the like, that a subject closer to the camera is basically assumed as a dominant subject and is to be focused. Here, in a case where subjects are scattered at different distances within, for example, the expanded focus area (regions 0 to 5) in FIG. 12, if the autofocus control started from a near side toward a distant side, it can be assumed that the dominant subject at the near side was focused at a peak position at the near side of a focus evaluation value that had found first.
However, if the autofocus operation started from the distant side toward the near side, some subject at the distance would be focused at a first-found peak position of the focus evaluation value at the distant side, making an intended subject at the near side out-of-focus.
Further, the dominant subject is generally located at the center of an imaging region, and thus the focus area is first set at the center of the imaging screen. However, in some cases, the subject may be shot intentionally off the center of the screen. When the dominant subject is thus present any of left, right, upper or lower side of the center region, a conventional method was used such that a user once determines the position of the dominant subject at the center of the imaging region and pushes a release switch of a camera halfway to focus the dominant subject by the autofocus operation, and thereafter moves the camera to bring the dominant subject off the center while keeping the release switch half-pushed before pushing the release switch all the way for shooting.
However, such a method is complicated, and a user having no knowledge thereof would just shoot the dominant subject off the center of the screen as it is, resulting in a picture in which the background is focused whereas the subject is out of focus.
Further, even though the dominant subject is simply positioned at the center, if the background has a high contrast, the high frequency component, i.e. focus evaluation value, of the video signal of the background would be increased, resulting in a picture in which only the background is focused (this is referred to as a background-focused state).
In particular, it would be difficult to focus a small dominant subject based on the focus evaluation value obtained from the entire focus area, and hence would be unavoidable that the distant background having a higher focus evaluation value would be focused.